In the current PEM fuel cell state of the art, the bipolar plate—MEA electrical contact occurs via a gas diffusion layer. Typically, the GDL is a two layer composite material that consists of a woven or non woven material coated with a porous carbon powder-polymer mixture (MDL). When assembled in a fuel cell, the carbon fiber material faces the bipolar plate surface while the MDL faces an electrocatalyst layer of MEA. The main function of GDL is to uniformly distribute the reactants and electrons across the entire MEA surface, and to manage the liquid water in the catalyst layer. Because of the non optimized design, its role is very limited and unpredictable. As a result of the GDL dysfunctional operation, PEM fuel cells may have high performance loss and a short life. One of the main reasons for the non uniform operation is compressive force imposed on the GDL that is induced on fuel cell in order to increase the interfacial electrical contacts within its components and the GDL conductivity itself. Carbon fiber layers that are aligned in x-y plane of the GDL must be under constant force to minimize the electrical resistance. This compression is the highest at the areas where the lands of the anode and cathode bipolar plates overlap and the lowest in the areas where their flow channels intersect. Thus, the electrical conductivity of GDL is the highest in the areas that are in contact with lands and lowest above flow channels. Compression in turn has the opposite effects on reactant flow distribution and water management porous media. In the area with the highest compression the flow is the lowest since the pores in GDL lessen under force; however, at no or low compression spots, the pores stay unchanged allowing reactant to freely pass to MEA. Consequently, the highest current density is generated in the most active areas of MEA that are the edges of lands since they have both the highest concentration of reactant and the highest electrical conductivity. In addition, the capillary action of the GDL that manages liquid water at the catalyst layer is also affected with the change of the pore size. The pores under compression that decrease and get irregular geometry under compression will start to accumulate water and loose the water managing ability. Therefore, the combined effects of non uniform electrochemical and mechanical stresses will create overloaded areas in the polymer electrolyte membrane and lead to its premature failure. The present invention addresses and eliminates these deficiencies with a novel approach.
Three layer MEAs used in low (LT) or high temperature (HT) PEM fuel cells consist of ion exchange membrane with anode and cathode catalyst layers coated on the opposing faces. The active catalyst area of MEA is typically surrounded with uncoated membrane used to seal fuel cell. In the current state of the art of LT or HT technology, the main components of catalyst coating are platinum catalyst (Pt) supported on carbon powder (Pt/C) and ion exchange polymer. Typically, the polymer is as same as the one used for making membrane. The coating is usually made by mixing the components into ink that is then applied onto a membrane surface using standard techniques well known in thin film technology. For any type of PEM hydrogen/air fuel cell, typical Pt loading for combined anode and cathode electrodes is a minimum 0.6 mg/cm2. This catalyst amount when applied as a coating has only 30% electrochemically accessible surface area in comparison to the Pt/C catalyst powder. Yet, during operation, the initial catalyst surface area further decreases to ˜10% due to the Pt particle growth via dissolution and recrystallization, and agglomeration. Therefore, the catalyst amount deposited in MEAs can decrease by 90% if the way of the catalyst deposition is optimized. The main benefits of such advancement would be the MEA cost reduction. Numerous analyses predict that the Pt cost will predominate in the MEA cost even in their mass market production mode.
Proton conducting membranes used in current PEM fuel cell technology are made of acid based polymers. For instance, sulfonated poly tetra fluoro ethylene (PTFE) polymer known under brand name Nafion® is typically used for LT fuel cells while phosphoric acid doped poly benzyl imidazol (PBI) employed in Celtec® MEAs is used for HT fuel cells. Proton conductivity in competing LT and HT membranes are liquid media dependant. Vehicle type proton conductivity mechanism of Nafion® is enabled by the presence of liquid water in the membrane. High and low contents of water force Nafion® to swell and shrink, inducing mechanical stresses that result in premature membrane failure. In addition, absorbed water works as polymer plasticizer that increases the membrane creep especially at higher temperatures. Consequently, the membrane failure occurs at much faster rates as a result of the accelerated creep. Even though the proton conductivity in the HT membrane occurs via different mechanism (Grotthuss) it is still liquid dependant. In this membrane proton conductance occurs via intermolecular proton transfer within a dynamical hydrogen bond network formed by the association and dissociation of the phosphoric acid molecules. As highly hydrophilic, phosphoric acid constantly absorbs water from air. Diluted acid migrates out of the membrane, clogs gas pores and channels, and reacts chemically with metal catalyst. All these changes ultimately limit the HT MEA life and make the HT fuel cells extremely sensitive to the presence of liquid water.